sea water

How much salt is there in sea water? What concentration do you need before crystals start forming? What will those crystals look like? I am sure those are the kind of questions that keep you awake at night!

Of course this can easily assessed experimentally. On a visit to the University of Bergen’s Centre for Science Education just now, I was shown the result of such an experiment: A litre of water was mixed with 35 grams of salt to simulate sea water with its typical salinity. Below, you see what the beaker looked like after sitting out for three months.

You can see that salt crystals are forming at the walls of the beaker, but that their structure depends on depth below the initial water level (see the 1000 ml mark on the beaker).

When there is still a lot of water in the beaker, crystals look like ornate flowers. Then, the less water is left in the beaker, the more square the crystals become. And at the bottom of the beaker, you see the typical salt crystals you would expect.

Actually, even though they look like the kind of salt crystals I would expect, apparently someone who knows about crystallography commented that there must be other stuff in there than just cooking salt since the crystals don’t look the way they should. I need to read up on this! :-)

Anyway, this is an experiment that I want to do myself, so maybe in three months time there will be more pictures of this!

Thanks for a very nice lunch, Olaug, Frede, Andreas, Morven and Elin! Looking forward to working with you a lot more in the future! :-)

P.S.: with this blog post I am testing to blog pretty much “real time” from my mobile phone, so if you notice anything odd, please let me know!

As reported earlier, students had to conduct experiments and present their results as part of CMM31. Niklas chose to demonstrate the mediterranean outflow – warm and salty water leaving the Mediterranean and sinking to a couple of kilometer’s depth in the Atlantic Ocean.

Since I happened to be around, they allowed me to document the experiments and blog about it, but there is a great description, including a movie, to be uploaded on the webpages of the University Centre of the Westfjords.

When the guys were done with the experiment, I couldn’t help but suggest to tip the tank so that the densest water would spill back into “the Mediterranean”. Check out the movie below if you fancy playing!

Students acting out the process of sound being refracted towards the region of minimum speed.

We’ve been talking about refraction lately. Waves get bent in the direction of lower velocity. This holds for light and sound and even ocean waves. However, students find it conceptually difficult to understand why waves are being bent towards lower rather than higher speeds, so I came up with this very simple demonstration.

Students, arms joint, are acting as a wave crest. Students on the one side of the student chain are told to move very slowly, students on the other side are asked to move quickly towards the instructor. Everybody takes care to not hurt anybody, so if tension builds up in the chain, everybody has to react to reduce the tension. What happens is that the “wave crest” of students changes direction towards the side of the slowest motion.

Easy visualization and – since it involved students getting up, joining arms and doing something – also very memorable. Win – win!

Another easy example: When you are sliding on an icy road and your foot gets caught in grass or gravel or something on one side (== region of lower velocity), you start skidding towards the side with the obstacle, not towards the middle of the icy road.

As described in this post, I like to have students build “instruments” to measure the most oceanographic properties (temperature, salinity and density). I find that they appreciate oceanographic data much more once they have first-hand experience with how difficult it is to design instruments and make sense of the readings. Over the last two days I described the experiments for salinity and density, today it’s temperature.

Students building thermometers.

Measuring temperature is probably the most difficult of the three properties. Firstly, there are lots of technical difficulties to be overcome. How can we seal the mouth of the bottle around the straw in a way that it is really water tight? How much water do we have to fill in the bottle? Does it matter if there are air bubbles trapped? What if the water level when we fill the bottle is not visible because of the seal? If the straw is clogged up with modeling clay, will we still be able to use it in the instrument? How long does the straw have to be above the seal in order to avoid water spilling out when the temperatures we try to measure become too hot?

Then, there are many problems connected to the actual measurement. If we lift up the thermometer (and hence squeeze the plastic bottle) – how does that influence our reading? Since we have half a liter of water in the thermometer, are we actually measuring the temperature of the water sample, or are we influencing it while trying to measure? How do we come up with a scale for our temperature measurements had I not supplied (mercury-free) thermometers to calibrate the new thermometer with? So many questions to think about and discuss!

As described in this post, I like to have students build “instruments” to measure the most oceanographic properties (temperature, salinity and density). I find that they appreciate oceanographic data much more once they have first-hand experience with how difficult it is to design instruments and make sense of the readings. Today I’m presenting two groups that focused on salinity, while yesterday’s group was measuring density.

Students evaporate water to measure the salinity of a water sample.

The students in the course I currently teach were determined to not only evaporate some water to qualitatively look at how much salt was dissolved in the sample, they wanted to do it right. So they set out to measure the vessel, the sample and the remaining salt. But since measuring salinity is really pretty difficult, they ran into a couple of problems. First – my scales were nowhere near good enough to measure the amount of water they could fit into the evaporation cup with any kind of precision. Second, even the amount of water that they could fit took a lot longer to evaporate (or even boil) than anticipated. Third, they realized that even though they could see salt residue in the end, this might not be all the salt that had been there in the beginning, plus there was grime accumulating at the base of the cup, so weighing the cup in the end might not be the best option. But they still learned a lot from that experiment: For example that once the (small quantity of) water was boiling, it became milky very quickly and then turned to crystallized salt almost instantly. Or that in order to use this method, a tea candle is not as suitable as a heat source as a lighter (and there might probably even be even better ones out there).

P.S.: In this course, none of the groups set the wooden tongs on fire! :-)

A very good introduction to the concept of salinity is the “tasting sea water” activity. Last time I ran that activity, students were very quick to correctly connect the samples with the correct sampling locations without much discussion going on. This time round, though, there was a lot of discussion. Students quickly sorted samples in order of increasing salinity, but there was no agreement to be found on whether the Baltic or the Arctic should be fresher. Since I only pointed to a location and didn’t specify the depth at which the sample had been taken, some students argued that the Arctic was very fresh at the very top, whereas the Baltic was brackish. Others said that the Baltic was a lot fresher than any oceanic location.

Students tasting four different samples of “sea water” with salinities corresponding to Arctic sea ice, the Baltic sea, the open ocean and the Mediterranean. Samples have to be associated with locations on a map.

In another group, there was a big discussion going on about how in marginal seas, evaporation or precipitation can dominate.

It is always great to see how much you can discuss and learn from an activity as simple as this one!

Fresh water and salt water ice have very different structures as I already discussed in this post.

Fresh water ice (on the left) and salt water ice (on the right).

In the image above you see that the structures are very different. Whereas fresh water ice is clear and transparent, salt water ice has a porous structure and is milky.

Investigating fresh water and salt water ice cubes in class. Already in this photo the difference is clearly visible, and it is even more obvious when you pick up the cups and look at the ice cubes from the side.

The pores can be made visible by dropping dye on the ice cubes, as we did in class on Tuesday. For salt water ice, dye penetrates into the ice cube along the brine channels; the ice cube seems to be soaking up the dye like a sponge and becomes colored through and through. In case of the fresh water ice, dye cannot penetrate because the crystal structure is so regular and tight, and the dye just comes off the ice.

Different didactical settings in which the “ice cubes melting in fresh and salt water” experiment can be used.

In part 1 and 2 of this series, I showed two different ways of using the “ice cubes melting in fresh water and salt water” experiment in lectures. Today I want to back up a little bit and discuss reasons for choosing one over the other version in different contexts.

Depending on the purpose, there are several ways of framing this experiment. This is very nicely discussed in materials from the Lawrence Hall of Science (link here), too, even though my discussion is a little different from theirs.

1) A demonstration.

If you want to show this experiment rather than having students conduct it themselves, using colored ice cubes is the way to go (see experiment here). The dye focuses the observer’s attention on the melt water and makes it much easier to observe the experiment from a distance, on a screen or via a projector. Dying the ice cubes makes understanding much easier, but it also diminishes the feeling of exploration a lot – there is no mystery involved any more.

Demonstration of melting ice cubes. The melt water is clearly marked by the dye. This makes it a good demonstration, but diminishes the satisfying feeling of discovery by the observer, because the processes are clearly visible right away rather than having to be explored.

2) A structured activity.

Students are handed (non-colored) ice cubes, cups with salt water and fresh water and are asked to make a prediction about which of the ice cubes is going to melt faster. Students test their hypothesis, find the results of the experiment in support with it or not, and we discuss. This is how I usually use this experiment in class (see discussion here).

The advantage of using this approach is that students have clear instructions that they can easily follow. Depending on how observant the group is, instructions can be very detailed (“Start the stop watch when you put the ice cubes in the water. Write down the time when the first ice cube has melted completely, and which of the ice cubes it was. Write down the time when the second ice cube has melted completely. …”) or more open (“observe the ice cubes melting”).

3) A problem-solving exercise.

In this case, students are given the materials, but they are not told which of the cups contains fresh or salt water (and they are instructed not to taste). Now students are asked to design an experiment to figure out which cup contains what.

This is a very nice exercise and students learn a lot from designing the experiment themselves. However, this also takes a very long time, more than I can usually afford to spend on experiments in class. After all, I am doing at least one hands-on activity in each of the lectures, but am still covering the same content from the text book as previous lecturers who used their 180 minutes per week just lecturing. And I am considering completely flipping my class room, but I am not there yet.

4) An open-ended investigation.

In this case, students are handed the materials, knowing which cup contains fresh and salt water. But instead of being asked a specific question, they are told to use the materials to learn as much as they can about salt water, fresh water, temperature and density.

As with the problem-solving exercise, this is a very time-intensive undertaking that does not seem feasible in the framework we are operating in. Also it is hard to predict what kind of experiments the students will come up with, and if they will learn what you want them to learn. On the other hand, students typically learn much more because they are free to explore and not bound by a specific instruction from you.

When preparing “sea water samples” for class, it is always astonishing to me how much salt I have to add for normal open-ocean salinities. Time and time again it looks like it should be way too much, but then when tasting it, it tastes salty, but like the ocean and not like brine.

A teaspoon full of salt corresponds to approximately 5 grams. That means that for typical open-ocean salinities, you have to add 7 teaspoons full of salt to a liter of water.

Since it is still astonishing to us, Pierre and I thought, it would probably be a good thing to show to our students. 0.18 teaspoon full of salt corresponds to only 1 gram of salt (averaged over several non-scientific internet sources, but well within the measurement error of my kitchen scales [and yes, I know the trick of measuring the weight of several spoons and then dividing by the number, but thanks!]).

What I want to do in the lecture is have the students estimate how much salt they need for a 35 psu liter of water. And not estimate by weighing (because I want each of the students to be able to touch the salt, but at the same time don’t want salt all over the lecture theatre), but visually estimate.

10 grams of salt in a little plastic jar.

The little jar in the picture above contains 10 grams of salt. So in order to have students estimate how much salt they would need for a liter of 35psu water, we filled 12 of those little jars with 10 grams each and handed them to the students. Obviously we didn’t tell the students how much salt was contained in a jar!

12 x 10 grams of salt. It does look like a lot more, doesn’t it?

Knowing that there are 10 grams of salt in each of the jars, it is pretty obvious that we need three and a half of those little jars for 35 grams of salt. When we did this in the lecture on Tuesday – and again, the students were not told how much salt was in one jar! -, the first person who answered guessed “four”. And then someone actually said “three and a half”. Oh well, lucky guess or great skill? I was hoping for answers like “maybe one of those jars”, because that would be closer to my own intuition. I guess next time I’ll be framing it differently. Maybe use something with one liter volume and put 35 grams in it? Or ask them to tell me in teaspoons? Does anyone have a good idea that they would like to share with me?

Sea ice and fresh water ice have distinctly different properties that can easily be investigated even in big class rooms.

In “on how ice freezes from salt water” I talked a bit about how dye was rejected when I tried to produce colored ice cubes for another experiment. But even non-colored ice that were made out of fresh water or salt water shows distinctly different structures.

Ice formed from fresh water (on the left) and salt water (on the right). Note the small pores in the salt water ice cube – those are the channels that form when brine is rejected.

On the left, you see that the surface is very smooth apart from a couple of cracks. The red food dye that was dripped on the ice cube comes right off, like water off a duck’s back. On the right, the food coloring is not rolling off, instead it is creeping into all the little brine channels, hence nicely showing a web of pores all throughout the ice cube.